This invention relates generally to magnetic resonance imaging (MRI), and more particularly the invention relates to spectroscopic imaging of the brain, for example, while suppressing interfering lipid signals and water signals.
Magnetic resonance imaging (MRI) is a non-destructive method for the analysis of materials and represents a new approach to medical imaging. It is completely non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radiofrequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
Numerous studies have shown that .sup.1 H spectroscopic imaging can potentially provide valuable clinical information for diagnosing and evaluating metabolic processes in normal and diseased brain. However, a number of technical difficulties must be overcome. Water suppression throughout a large region of interest (ROI) is difficult due to both B.sub.0 and B.sub.1 inhomogeneities. In addition, intense lipid signals from subcutaneous fat surrounding the skull must be eliminated to avoid overwhelming the much smaller desired metabolite signals.
Known successful techniques for spectroscopic imaging use a series of spatially selective RF pulses (one along each spatial axis) to excite a rectangular region which is carefully positioned to lie wholly within the brain. The excited ROI is then phase encoded along two axes to generate a 3-dimensional data set which can be processed to form images of the various metabolites. Without the ROI restrictions, the relatively coarse locations provided by the phase encoding is typically insufficient to prevent unwanted fat signals from severely contaminating the desired spectra, even for voxels deep within the brain.
While limiting the ROI to an interior volume is an effective method of fat suppression, there are a number of limitations. Locating a rectangular ROI within the skull involves a fundamental tradeoff between the achievable lipid suppression and the size of the excited volume. While a large ROI is desirable, care must be taken to keep the edges of the volume away from the skull. Thus, examining regions close to the skull is particularly difficult and usually requires a significantly smaller ROI. One alternative prior art method uses multidimensional pulses to excite an elliptical ROI which fits more easily within the skull. However, any method which uses spatially selective RF pulses to eliminate the subcutaneous fat will suffer from imperfect slice profiles and finite slice transition widths.